To support applications with rigorous QoS requirements in a Collision Sense Multiple Access/Collision Avoidance (CSMA/CA) there must be some sort of control mechanism to insure one or more of the following attributes of a session, (1) availability of adequate bandwidth; (2) low jitter, i.e., ±5 ms; and (3) low latency, i.e., 5 ms nominal, depending upon the needs of the supporting applications.
The problem with providing a control mechanism is that ad hoc networks are basically anarchic: there is no central control, and any program or system attempting to implement a centralized control function will compete with other programs each time the control program attempts to seize the channel and establish a QoS environment.
Having determined that some form of control is desirable to resolve the problem of rigorous QoS, the question becomes one of implementation of control in an ad hoc network, i.e., one with no central control. While there are many issues involved in implementing a control mechanism, the first issue is how to insure that the control mechanism can gain control of the medium in a reliable and timely fashion.
Ad hoc networks generally implement a form of CSMA/CA to allow a device to gain control of the channel. Some networks, e.g., 802.11, allow a QoS Manager, usually identified as a point coordination function (PCF), or a hybrid coordination function (HCF) in 802.11e, to assert control after a shorter interframe gap than that allocated to other devices, thereby insuring that the QoS manager will gain control during the next contention interval. It could be argued that 802.11 is, in its fundamental form, an ad hoc network, termed an independent basic service set (IBSS), which has a distribution/control mechanism, the PCF, overlaid on top of it. When a PCF is present, the network is termed a basic service set (BSS), and is considered an infrastructure network, not an ad hoc network.
Other ad hoc networks, e.g., HomePlug, provide a multi-level priority mechanism so that only those devices asserting the highest active priority level actually contend during the contention interval. In these networks, a QoS manager must compete on an even footing with other devices at the same priority level for control of the channel. HomePlug is a registered trademark of the HomePlug Alliance, a California Corporation.
The problem for a QoS manager is that having to compete on an equal footing can have a serious impact on the levels of latency and jitter that the QoS manager can provide. Depending upon the maximum duration that a device can hold a channel once the device has seized the channel, a QoS manager competing on an equal footing with other devices may have to wait so long before gaining control of the channel that it would completely miss one of the periodic intervals in which it is committed to deliver traffic.